The number of CGG repeats and the methylation status of the CpG island of FMR1 (Fragile X Mental Retardation Gene 1) influences the phenotype in fragile X syndrome. Therefore, accurate determination of these two parameters is crucial for interpretation of prenatal DNA testing and appropriate counseling. Methylation in FMR1 is associated with both normal (X-inactivation) and aberrant processes (expansion of the CGG trinucleotide repeat segment). In order to assess the appropriateness of chorionic villus sampling (CVS) for prenatal diagnosis of fragile X syndrome, paired (female) fetal and extraembryonic samples from pregnancy terminations ranging from 9 to 12 week gestation are under study, and methylation status of the FMR1 region on the inactive X determined by Southern blot analysis. Comparison of levels of methylation between CVS and fetal samples, and their relationship to gestational age are the objects of these investigations, which may have an impact on recommendations for prenatal diagnosis of FMR1, particularly with regard to CVS utilization. for prenatal diagnosis of FMR1.